1336
Inorg. Chem. 1998, 37, 1336-1340
Magneto-Structural Correlation Studies of A Ferromagnetically Coupled Dinuclear Vanadium(IV) Complex. Single-Crystal EPR Study Murugesan Velayutham, Babu Varghese, and Sankaran Subramanian*,† Regional Sophisticated Instrumentation Centre, Indian Institute of Technology, Madras, Chennai-600 036, India ReceiVed June 17, 1997 The dimer potassium dioxo(citrato)vanadate(IV)-6-water, K4{VO[O2CCH2C(O)(CO2)CH2CO2]}2‚6H2O, was prepared by the reaction of citric acid and metavanadate in a neutral solution. The complex crystallizes in the space group P1h with unit cell parameters a ) 8.474(5) Å, b ) 8.902(7) Å, c ) 9.596(9) Å, R ) 71.50(6)°, β ) 70.81(9)°, γ ) 87.45(2)°, V ) 647.1 Å3, and Z ) 1. The dimeric anion contains a centrosymmetric planar four-membered V2O2 ring with the bridging oxygens derived from the hydroxyl groups. The configuration of the anion is anti-coplanar. A frozen solution EPR spectrum with zero-field splitting and 15 line hyperfine patterns on the parallel and perpendicular features characteristic of two equivalent vanadium atoms confirms the presence of a spin triplet. This is further supported by the presence of half-field “forbidden transition” at g ≈ 4. A broad line with half-field forbidden transitions in the powder EPR spectra at X,Q-bands of the neat sample and at different temperatures shows the presence of a spin triplet ground state and a very weak intermolecular dipolar interaction. From anisotropic exchange contributions to D the magnitude of the derived exchange integral Jxy,x2-y2 is 56 cm-1. UV-visible, ESCA, and theoretical BVS studies support the proposed molecular and electronic structure of the complex.
Introduction The first biologically important role assigned to vanadium was as an inhibitor of Na+, K+-ATPases and nucleases.1 Vanadium is present in all mammalian tissues at concentration of about 10 µM or less. Vanadium plays a significant role in the reduction of N2 to NH3.2 Citric acid is an important compound in biological processes. For example, citrates appear at about 0.1 mM in blood plasma and occur at 0.3 wt % in teeth and bone. They regulate some fundamental physiological processes and are intermediates in carbohydrate metabolism, e.g., in the “Krebs cycle”.3 Vanadium complexes are used as restriction enzymes to break the simple nucleotides and their polymers, i.e. DNA and RNA, and also used as anticancer drugs.4 A recent work has focused on the insulin-mimicking properties of vanadium compounds.5-7 The anionic complexes of VO2+ ion with R-hydroxy carboxylate anions in which both hydroxyl and carboxyl protons are removed exhibit novel structural, optical, and magnetic properties. Citric acid, acting as a tetradentate ligand, readily forms polynuclear complexes with numerous metal ions.8 Citric acid under †
Present address: Radiation Biology Branch, National Cancer Institute, National Institutes of Health, Bethesda, MD 20892. E-mail:
[email protected]. Fax: (301) 480 2238. (1) Cantley, L. C.; Josephson, L.; Warner, R.; Yanagisawa, M.; Lechene, C.; Guidotti, G. J. Biol. Chem. 1977, 252, 7421. (2) Rehder, D. Angew. Chem., Int. Ed. Engl. 1991, 30, 148. (3) Djordjevic, C.; Lee, M.; Sinn, E. Inorg. Chem. 1989, 28, 719. (4) Dean, N. S.; Mokry, M.; Bond, M. R.; O’Conor, C. J.; Carrano, C. J. Inorg. Chem. 1996, 35, 2818. (5) Codd, R.; Hamblay, T. W.; Lay, P. A. Inorg. Chem. 1995, 34, 877. (6) Sun, Y.; James, B. R.; Rettig, S. J.; Orvig, C. Inorg. Chem. 1996, 35, 1667. (7) Hanson, G. R.; Sun, Y.; Orvig, C. Inorg. Chem. 1996, 35, 6507. (8) Kiss, T.; Buglyo, P.; Sanna, D.; Micera, G.; Decock, P.; Dewaele, D. Inorg. Chim. Acta 1995, 239, 145.
different experimental conditions directs the metal atoms to different oxidation states.9 A more detailed understanding of the nature of vanadium and biologically important ligands such as citric acid interactions necessitates a detailed knowledge of the coordination environment and oxidation state of the metal(s) within the systems. The study of magnetic properties of molecular compounds is of relevance for the understanding of magnetically coupled active sites in metalloproteins as well as their potential applications as magnetic materials. Most dimeric oxovanadium(IV) complexes exhibit an antiferromagnetic exchange interaction while a few complexes are reported to yield a ferromagnetic interaction.10 By studying the molecular, electronic structures, and magnetic properties using different instrumental techniques, it is possible to design ligand environments to introduce specific magnetic properties to dinuclear d1-d1 systems containing two oxovanadium centers. We report in this paper the synthesis, molecular and electronic structures, magnetic properties, and physicochemical properties of the complex K4[VO(cit)]2‚6H2O (cit ) citrate ion). Experimental Section Preparation of K4[VO(cit)]2‚6H2O. Ammonium metavanadate (10 mmol) and an excess of citric acid (15 mmol) were dissolved in water, and the solution was refluxed for 0.5 h, adjusted to neutral pH by 10% potassium hydroxide, and stirred for several hours. The dark blue mixture was filtered and evaporated, and the solid formed was colletced and recrystallized to give a blue solid. Physical Measurements. UV/vis spectra were recorded on an Shimadzu UV-3100 spectrometer. All the EPR spectra were recorded with a Varian E112 spectrometer (X- and Q-band frequencies) at 77 (9) Wright, D. W.; Humiston, P. A.; Orme-Johnson, W. H.; Davis, W. M. Inorg. Chem. 1995, 34, 4194. (10) Plass, W. Angew. Chem., Int. Ed. Engl. 1996, 35, 627.
S0020-1669(97)00746-5 CCC: $15.00 © 1998 American Chemical Society Published on Web 03/06/1998
A Ferromagnetically Coupled Dinuclear V(IV) Complex
Inorganic Chemistry, Vol. 37, No. 6, 1998 1337
Table 1. Crystal and Structure Refinement Data of K4[VO(cit)]2‚6H2O empirical formula fw temp wavelength space group unit cell dimens vol Z D(calcd) abs coeff R(Fo)a Rw(Fo2)a
C12H20K4O22V2 774.56 293(2) K 1.54184 Å P1h (No. 2) a ) 8.474(5) Å, R ) 71.51(7)° b ) 8.903(8) Å, β ) 70.82(5)° c ) 9.597(6) Å, γ ) 87.45(7)° 647.1(8) Å3 1 1.988 g cm-3 120.76 cm-1 0.0530 0.1503
a R(Fo) ) [∑||Fo| - |Fc||/∑Fo]. Rw(Fo2) ) {∑[w(Fo2 - Fc2)]2/ ∑[w(Fo)2]2}1/2, where w ) 1/[σ(Fo2)2 + (0.1130P)2 + (1.419P)] and P ) [max(Fo2,0) + 2Fc2]/3.
and 300 K. Variable-temperature (20-293 K) EPR was recorded using a CTI-Cryogenics set up. ESCA measurements were carried out using a VG Escalab MK II spectrometer under a vacuum of 10-9-10-10 Torr. Bond valence sum analyses were also carried out for the complex. X-ray Crystallography. A crystal of 0.3 mm × 0.3 mm × 0.2 mm size was mounted at the tip of a glass fiber attached to an EnrafNonius CAD4 diffractometer with Cu KR radiation. A total of 25 reflections with 40° < 2θ < 60° were collected through a random search routine and indexed using the principle of shortest vectors followed by least-squares refinement. A total of 2306 reflections with 2° < θ < 70° were collected, with the ω-2θ scan mode adopted for data collection, of which 2086 reflections had intensities with I > 2σ(I). Two reflections whose intensities were monitored once every 1 h showed no decay of crystal during data collection. The data were corrected for Lorentz and polarization effects. The absorption correction was effected using ψ scan data. The structure was solved by direct methods using the SHELX-86 computer program and refined using SHELXL-93. All non-hydrogen atoms were refined with anisotopic thermal parameters. Hydrogen atoms were geometrically fixed, and a riding model refinement was effected. The final difference Fourier map showed 1.169 e/Å3 as the highest peak. The final residual factors are R(Fo) ) 0.053 and wR(I) ) 0.15. The quantity minimized for leastsquares refinement is ∑w(Fo2 - Fc2)2, where w ) 1/[σ(Fo2)2 + (0.1130P)2 + (1.419P)] and P ) [max(Fo2,0) + 2Fc2]/3. Table 1 presents the crystal and structure refinement data in brief.
Results and Discussion Description of the Structure. The crystals separated from the products of the reaction between metavanadate and citric acid have 1:1 V:citrate mole ratio. Figure 1 shows the molecular structure of the complex. The complex crystallizes in the space group P1h with one molecule per asymmetric unit. The crystal structure comprises discrete potassium cations, water molecules, and centrosymmetric dimeric anions. Selected bond lengths and bond angles are given in the Table 2. The presence of two coordinated bridging citrate ions makes the coordination polyhedron distort significantly from octahedral and also makes the V---V distance one of the shortest for dinuclear vanadyl complexes.11 The chelation of the deprotonated hydroxy and carboxylic groups of the citrate ion leads to the presence of two six-membered rings and one five-membered ring, perhaps stabilizing the overall dimer moiety. The core [VO(µ2-O)]2 forms a centrosymmetric planar structure. The two vanadium atoms are in the basal plane, and the two citrate ions are sitting above and below the plane. The two vanadium-oxygen bond lengths involving the bridging (11) Rosenberger, C.; Schrock, R. R.; Davis, W. M. Inorg. Chem. 1997, 36, 123.
Figure 1. Molecular structure of K4[VO(cit)]2‚6H2O. (ORTEP representation of the molecule depicting 50% propability ellipsoids.) Table 2. Selected Bond Lengths (Å) and Bond Angles (deg) for K4[VO(cit)]2‚6H2Oa V-O(1) V-O(2) V-O(4) V-V#1
2.171(3) 2.041(3) 2.022(3) 3.301(3)
O(4)-V-O(1) O(3)-V-O(4) O(1)#1-V-O(1) O(5)-V-O(3) O(2)-V-O(1) O(3)-V-O(2) a
100.30(13) 87.55(14) 74.60(13) 100.6(2) 75.78(12) 90.0(2)
V-O(1)#1 V-O(3) V-O(5) V-O(1)-V#1 O(1)#1-V-O(4) O(5)-V-O(4) O(3)-V-O(1) O(5)-V-O(1)#1 O(1)#1-V-O(2) O(5)-V-O(2)
1.977(3) 1.992(3) 1.608(3) 105.39(13) 87.91(13) 95.8(2) 80.89(12) 105.5(2) 92.70(13) 88.2(2)
#1: -x,-y,-z.
oxygens are almost equal. This shows the absence of mixed valence (V, III) in the dimer. The VOV angle is 105.39°, which shows that the σ-hybridization of bridging oxygen is near sp3.12 The σ-hybridization of bridging oxo and hence the bridge angle and V---V separations are subject to major control by the mutual steric requirements of the metal coordination spheres. The principle presented by Parai-Koshits says that when a structure with a center of symmetry is one of several differing very little in energy, it is usually found that the centrosymmetric structure is favored by the crystal packing.13 The complex anion has a crystallographically imposed center of symmetry, and the molecular symmetry is Ci. Vanadium complexes containing a [VO(µ2-OR)]24- ion have five possible cofigurations; they are anti-orthogonal, anti-coplanar, syn-orthogonal, syn-coplanar, and twist configurations.10 Many dinuclear vanadium complexes have been reported for the two orthogonal cofigurations. One example each for anti-coplanar and the twist configuration has also been reported,10 but a dinuclear complex with a syncoplanar configuration has not yet been synthesized. From the single-crystal XRD, the present complex is a second example with an anti-coplanar configuration (see Figure 1). Indeed, this is the first example of an ionic vanadyl complex with the anticoplanar configuration. This complex contains σ and π bonding ligands and is termed class II with equivalent geometries at the vanadium centers.14 This is supported by the EPR studies; Vide (12) Dean, N. S.; Bond, M. R.; O’Connor, C. J.; Carrano, C. J. Inorg. Chem. 1996, 35, 7643. (13) Porai-koshits, M. A. Zh. Neorg. Khim. 1968, 13, 1233.
1338 Inorganic Chemistry, Vol. 37, No. 6, 1998
Velayutham et al. spectrum could be simulated satisfactorily with the parameters of g| ) 1.95, g⊥ ) 1.98, A| ) 83 G, A⊥ ) 25 G, and D ) 0.065 cm-1. The half-field transitions for the frozen solution spectra at 77 K were very weak with hyperfine structure, and there is uncertanity in finding the Bmin. The zero-field splitting tensor for exchange-coupled S ) 1/2 ions contains contributions due to magnetic dipole-dipole and exchange interactions:18
D ) Ddip + Dex
(4)
The dipolar part of the zero-field-splitting tensor can be calculated by using the formula of Smith and Pilbrow,19 which has a very simple form for a centrosymmetrical dimer:
DRγdip ) 1/2gRgγ(δRγ - 3σRσγ)β2/r3 Figure 2. X-band EPR spectrum of frozen aqueous solution at 77 K.
infra. Waters of crystallization are distributed throughout the cell and together with various anionic oxygens make the potassium cations six coordinate. On the basis of the XRD data, theoretical BVS calculations were carried out;15 the observation of the +4 oxidation state for both vanadiums is consistent with the charge requirements of the molecule. Electronic Spectra. The band at 564 nm corresponds to a metal d-d transition. The highly intense peak at 303.5 nm corresponds to a ligand transition. The broad and low intensity appearance of absorptions from 600 to 900 nm may be due to metal d-d transitions. EPR. The EPR spectra of a spin-triplet state are descibed by the spin Hamiltonian16
H ) βB h gSh + ShDSh + hI ASh
(1)
where Sh ) Sh1 + Sh2 is the total spin of the system, Sh1 and Sh2 being the individual spins of the centers 1 and 2, D, the zerofield tensor, and A, the hyperfine coupling constant tensor. In a coordinate system where D is diagonal, one can write
H ) βB h gSh + D[Shz2 - 1/3S(S + 1)] + E(Shx2 - Shy2) (2) where D ) (2Dzz - Dxx - Dyy)/2 and E ) (Dxx - Dyy)/2. The D value can be extracted from the half-field transition through the relationship17
Bmin2 ) 1/g2β2[h2ν2/4 - (D2 + 3E2/3)]
(3)
where B is magnetic field, h is Planck’s constant, and ν is the operational frequency. Solution Spectra. The aqueous frozen solution spectrum of the complex is shown in Figure 2. The spectrum centered at 3200 G consists of the overlapping low- and high-field parallel and perpendicular components each with a 15 line hyperfine structure, with the high- and low-field parallel lines separated by 2D. The half-field, ∆Ms ) (2, transitions are clearly seen at 1600 G with some resolved nuclear hyperfine structure. The (14) Schulz, D.; Weyhermuller, T.; Wieghardt, K.; Nuber, B. Inorg. Chim. Acta 1995, 240, 217. (15) Thorp, H. H. Inorg. Chem. 1992, 31, 1585. (16) Abragam, A.; Bleaney, B. Electron Paramagnetic Resonance of Transition Ions; Clarendon Press: Oxford, England, 1970. (17) Belford, R. L.; Chasteen, N. D.; So, H.; Tapscott, R. E. J. Am. Chem. Soc. 1969, 91, 4675.
(5)
The σi variables are the direction cosines between the vector joining the two metal ions and the direction of gi (i ) x, y, z); δij is Kronecker’s δ function. gz is fixed along the VdO bond direction, gx is approximately along the O3 bond, and gy is perpendicular to the gx and gz directions. The calculated tensor Ddip is expressed as follows:
-0.0010 -0.0067 -0.0325 Ddip ) -0.0067
0.0216 -0.0089
-0.0325 -0.0089 -0.0201 Diagonalization yields Dxxdip ) 0.0234, Dyydip ) 0.0233, and Dzzdip ) -0.0463 cm-1. By using these values, and eq 2, the calculated Ddip ) -0.0696 and Edip ) 0 cm-1. From the simulated frozen solution spectrum the D value is 0.065 cm-1. Since the symmetry of the [V(O)2V] bridge is high, the zerofield-splitting tensor can be treated as axially symmetric. The experimental D value has either the + or - sign. The components of the experimental D value are Dxxexpt ) -0.0216 cm-1, Dyyexpt ) -0.0215 cm-1, and Dzzexpt ) (0.0431 cm-1. From the Dijexpt and theoretically calculated Dijdip values the DijEx values are calculated and given as DxxEx ) -0.0448 or -0.0018 cm-1. DyyEx ) -0.0449 or -0.0018 cm-1, and DzzEx ) 0.0894 or 0.0032 cm-1 depending up on either of the sign considerations. The exchange part of the zero-field splitting is due to the interaction of the two V4+ ground-state wave functions, which are mainly of dxy character. Contributions of the excited states dxy, dxz, and dyz are admixed via LS coupling, however. The components of the corresponding Dex tensor can be expressed in terms of the exchanged integrals involved Jxy,j (j ) x2 - y2, xz, yz):18
Dxxex ) 1/8(∆gx)2Jxy,xz Dyyex ) /8(∆gy)2Jxy,yz Dzzex ) 1/32(∆gz)2Jxy,x2-y2 (6)
1
DEx is a traceless tensor expressed in terms of a nontraceless tensor Dex. Here the assumption is Jxy,xz and Jxy,yz are small with respect to Jxy,x2-y2; i.e., Dxxex and Dyyex must also be small; then
Dzzex ) 1/48(∆gz)2Jxy,x2-y2
(7)
The calculated exchange integral Jxy,x2-y2 is 1569 or 56 cm-1, of which the second value is reasonable. The magnitude of (18) Ozarowski, A.; Reinen, D. Inorg. Chem. 1986, 25, 1704. (19) Smith, T. D.; Pilbrow, J. R. Coord. Chem. ReV. 1974, 13, 173.
A Ferromagnetically Coupled Dinuclear V(IV) Complex
Inorganic Chemistry, Vol. 37, No. 6, 1998 1339
Figure 4. EPR spectrum of the single-crystal orientation at Q-band.
Figure 5. Plot of θ vs magnetic field in the plane of maximum 2D separation.
Figure 3. Polycrystalline EPR spectra at 300 K: (a) X-band; (b) Q-band.
exchange integral is small, and hence, the anisotropic exchange interaction cotribution to the zero-field tensor is also not significant. It shows that the interaction between the two metal centers is almost completly dipolar in nature. Powder Spectra. The X- and Q-band EPR powder spectra of the complex were recorded at room temperature. Figure 3 shows the polycrystalline EPR spectrum at X- and Q-bands. The X-band spectrum shows broad structured bands with no detectable hyperfine structure at g ) 1.9816. The peak to peak separation was found to be 900 G, with a total spread of 2500 G. The Q-band spectrum shows slightly resolved peaks, and the nature of the peaks is the same as in X-band. From the molecular structure based on XRD and powder EPR at different frequencies, the complex is found to be a ferromagnetically coupled dinuclear metal center. The powder spectra of the neat compound are further subject to intermolecular magnetic interactions, and hence, the fine and hyperfine structures are not resolved. The broad band at half-field from the forbidden ∆Ms ) (2 transitions occur at 1650 and 6400 G at X- and Q-band, respectively. The spectral data are consistent with an S ) 1 spin system that exhibits a small zero-field splitting, as typically observed for other dimeric vanadium(IV) complexes with intramolecular interactions.10 The 77 K powder EPR spectra recorded at X-band show that there is a slight increase in the signal intensity. As mentioned above, at room temperature, the peaks are not resolved. These observations are indicative of intramolecular ferromagnetic interactions in the exchange-coupled d1-d1 systems and thus of a triplet ground state. The D value can be extracted from eq 3, where Bmin ) 1650 G, ν ) 9.4 GHz, and g ) 1.9816; we obtain D ) 0.0640 cm-1
for the X-band powder spectrum at 300 K. This D value is more or less the same as the molecular D value. Due to the bulkiness of the citrate ion, the intermolecular interaction is considerably reduced. The individual zero-field transistions and hyperfine splitting in the powder spectrum are not resolved due to line broadening. Attributing D to the magnetic dipolar interaction between the two free electron centers in the complex, we can easily estimate the same from the crystal data. These are as follows: V---V distance ) 3.30 Å; angle between VdO bond V---V direction ) 37.72°. From the powder EPR g ) 1.9816 we can calculate the magnitude of this interaction along the VdO bond (θ ) 37.72°) or along the V---V direction (θ ) 0°). The corresponding values are 0.0207 or 0.0472 cm-1, respectively. The latter one is slightly higher than the experimental value from the powder and single-crystal data (next section). The maximum Dcalcd will occur at θ ) 0°. Single-Crystal Spectra. Figure 4 shows the X-band spectrum of an oriented single crystal of the complex. Angular variations of the g-values were measured with the magnetic field vector scanning three mutually orthogonal planes at both Xand Q-band frequencies. Figure 5 shows the plot of θ vs magnetic field in Q-band for a plane with maximum separation of 2D. In this plane the plot is identical for both the frequencies. Since Z ) 1, the EPR spectra also correspond to one magnetically distinct site. In most of the orientations two peaks with hyperfine structure are seen. In single crystals the spectra are due to a single orientation of the molecule and not due to a statistical distribution of orientations. Hence in single-crystal spectra the hyperfine structures are well resolved. In some of the orientations we are able to see the half-field transition. The plot follows the (1 - 3 cos2 θ) dependence. The maximum separation of 2D is 1360 G. This D value is in agreement with the isolated molecular D value from the frozen solution spectrum.
1340 Inorganic Chemistry, Vol. 37, No. 6, 1998 Variable-Temperature EPR. Variable-temperature polycrystalline EPR spectra were recorded from 20 to 293 K at X-band. Lowering of the temperature does not have much effect on the spectra except for a marginal increase in the intensity. Therefore, in the ground state the vanadium atoms are ferromagnetically coupled. Generally (µ-OH)2-bridged dinuclear vanadium complexes are antiferromagnetically coupled and a µ-O complex is diamagnetic.20,21 However, (µ-O)2 complexes are ferromagnetically coupled.10 A qualitative interpretation of the magnetic interactions in transition metal dimers is usually given by the well-known Goodenough-Kanamori rules, which are based on the interaction between pairs of natural magnetic orbitals.22 The unpaired electron in oxovanadium(IV) complexes generally resides in a metal-centered orbital with dxy character, with the oxo group oriented along the z-axis. For the complex with an anti-coplanar configuration, the orthogonality is certainly dependent on the structural parameters of the [VO(µ2-O)]2 core, and moreover, the direct overlap between the two magnetic orbitals is expected to be rather small. When V(O)2V is planar and the V---V distance is short, the direct overlap pathway will lead to an antiferomagnetic interaction.10 For a ferromagnetically coupled system the two magnetic orbitals are orthogonal to each other. In the present complex such a condition favors the observed ferromagnetic interaction. From EPR studies the unpaired electrons are found to be equally interacting with both the nuclei. The mechanism to explain the interaction of the two vanadium centers and the resulting ferromagnetic coupling is due to a “crossed interaction ” between singly occupied dxy orbital and empty dyz orbital at either transition metal center using the py orbital of a bridging oxygen atom.10 The dxy|py|dyz orbital overlap is depicted in the Figure 6. In systems where magnetic interactions are less than 0.3 cm-1 EPR spectroscopy is a much more sensitive tool than susceptibility measurements, and we get information at the molecular level. In the present system the major interaction is the intramolecular dipole-dipole coupling characterized by a D of about 0.065 cm-1 with negligible intermolecular magnetic coupling. Temperature-dependent EPR studies clearly show an (20) Toftlund, H.; Larsen, S.; Murray, K. S. Inorg. Chem. 1991, 30, 3964. (21) Wieghardt, K.; Bossek, U.; Volckmar, K.; Swiridoff, W.; Weiss, J. Inorg. Chem. 1984, 23, 1387. (22) Ginsberg, A. P. Inorg. Chim. Acta ReV. 1971, 5, 45.
Velayutham et al.
Figure 6. 6. Schematic representation of the “crossed interaction”.
increased intensity at lower temperatures establishing a ferromagnetic coupling for the ground state of the dimer. The molecules retain their integrity in solution, and the EPR spectra of the frozen solution clearly reveal the chemical equivalence of the two vanadium ions and the through-space dipolar coupling. ESCA. The ESCA peaks of carbon 1s and potassium 2p overlap with each other. The oxygen 1s peak occurs at 531 eV. The peak at 516 eV corresponds to vanadium 2p3/2 in the oxidation state of IV.23 This shows that both the vanadium atoms are in the same oxidation state. There is no indication for mixed valence of the metal centers. Conclusion This ionic complex is the first example of an edge-sharing octahedral oxovanadium(IV) dimer that exhibits a ferromagnetic interaction between the metal centers and has an anti-coplanar cofiguration. The magnetic interaction between the two metal center is mainly dipolar in nature. By understanding the magneto-structural correlation of the [VO(µ2-OR)]24- ion by spectroscopic techniques, it is possible to envisage specific magnetic properties of such systems through the choice of ligands employed. Supporting Information Available: Listings of crystallographic parameters, atomic positional and thermal parameters, anisotropic thermal parameters, and full bond distances and bond angles (10 pages). Ordering information is given on any current masthead page. IC9707467 (23) Ferrer, E. G.; Baran, E. J. J. Electron Spectrosc. Relat. Phenom. 1991, 57, 189.
